Story Highlights

Katee Sackhoff had to get physical to be just as intimidating onscreen as Vin Diesel.

Going the way of male superhero stars, the actress added 15 pounds of muscle to her already-athletic 5-foot-5 frame to play Dahl, the only woman in a crew of outer-space bounty hunters seeking Diesel's title antihero in the sci-fi tale Riddick (out Friday).

"It's probably the closest I've come to playing a man in my life!" the former Battlestar Galactica star says with a laugh. "She's incredibly strong, she's very, very, very tough, and there's not an emotional bone in her body. That's something I've never played before.''

While her bounty-hunting sniper is by far the manliest role she's had, "there's something incredibly sexy in the fact that she is completely unforgiving."

In addition to carrying around a massive gun "as big as I was," Sackhoff, 33, wanted to make sure she wouldn't look small compared to Diesel, former pro wrestler Dave Bautista and her other brawny co-stars.

She spent two months of weight training to transform her body, getting to the point where she was curling 35 pounds with each arm.

"I was lifting so heavy that guys thought I was insane," Sackhoff says. "I mean, I look big. (Dahl's) a girl who lives with guys, eats with guys and trains with guys, and going in with that mind-set and knowing the guys are almost all over 6 feet, I knew I needed to hold on to weight and make her bigger."

There was one problem: While her body likes to hold on to muscle, it doesn't tend to stay bulky. Sackhoff found she was losing mass once she started filming in Montreal, and when she couldn't find the peanut butter fudge she was eating back in Los Angeles to keep weight on, she had her manager ship her a huge block of the treat.

It was "definitely cool" to see herself look so tough on screen, but it was also hard "because I'm so feminine. I'm normally like, 'She looks so pretty! Look at her, she's so cute!' In that situation, you kind of have to throw vanity out the window because there's not a delicate feminine bone in Dahl's body."

Director David Twohy had Sackhoff work out with Diesel on an unseen backstory that would help flesh out her character onscreen. She decided that she was going to be a second-in-command to co-star Matt Nables' Boss Johns character and added some moments where she'd be at his side waiting for orders.

"It helped me find my character because what you realize is she's a phenomenal soldier and she does what she's told," Sackhoff says.

That was also around the time when her fashionista side emerged: Even though Dahl was one of the guys, the actress thought she should have a sexier wardrobe. The heavy blue material the bounty hunters wore looked unattractive on her with the extra muscle weight.

"I said (to Twohy), 'Knowing she's the sniper, we could put her in stretch pants, chaps and a corset. Are you good with that?' " she recalls.

"It was awesome. That's how Dahl ended up in a corset, which I justify saying that it helps keep her shoulders back when carrying heavy weapons."